The invention relates to a scandate cathode having a cathode body which comprises a matrix of at least a high-melting point metal and/or alloy with a barium compound at least in the matrix in contact with the matrix material, which compound can supply barium to the emissive surface by a chemical reaction with the matrix material.
The invention also relates to methods of manufacturing such a cathode and to an electron beam tube provided with such a cathode.
Cathodes of the type mentioned in the opening paragraph are described in the article "Properties and manufacture of top layer scandate cathodes", Applied Surface Science 26 (1986), pp. 173-195, J. Hasker, J. van Esdonk and J. E. Crombeen. In the cathodes described in this article scandium oxide (Sc.sub.2 O.sub.3) grains of several microns or tungsten (W) grains which are partially coated with either scandium (oxidation occurs during impregnation in the latter cathodes) (Sc) or scandium hydride (ScH.sub.2) (oxidation occurs during impregnation in the latter cathodes) are present at least in the top layer of the cathode body. The cathode body is manufactured by pressing and sintering, whereafter the pores are impregnated with barium-calcium-aluminate. In order to maintain the electron emission, the barium-calcium-aluminate supplies barium on the emissive surface by a chemical reaction with the tungsten of the matrix during operation of the cathode. To be able to realise a very high cathode load in, for example a cathode ray tube, it is important that a scandium-containing layer having a thickness of approximately one monolayer be formed on the cathode surface during impregnation by means of a reaction with the impregnating agent. As has been proved in experiments described in the above-mentioned article, the scandium-containing layer may be completely or partly removed by an ion bombardment which may occur in practice, for example during the manufacture of television tubes, which remova leads to detrimental consequences for the electron emission. Since Sc.sub.2 O.sub.3 is not very mobile the said scandium-containing layer cannot be fully regenerated by reactivation of the cathode. The described experiments have also proved that a regeneration which is sufficient for a complete recovery of the emission is not achieved. As compared with an impregnated tungsten cathode coated or not coated with, for example osmium, this may be considered as a drawback.